Meningitis

Meningitis is inflammation of the meninges and infection of the cerebrospinal fluid (CSF), a normally sterile compartment. The CNS is especially vulnerable to oedema because the rigid skull limits expansion; raised intracranial pressure can lead to herniation of the brainstem and compression of respiratory and cardiac centres. Causes include bacteria, viruses, fungi, and protozoa, as well as non-infectious causes such as lymphoma, subarachnoid haemorrhage, and drug reactions.

Bacterial meningitis (acute purulent meningitis) is caused by encapsulated commensals that colonise the upper respiratory, gastrointestinal, or urogenital tract. All bacteria causing acute meningitis have polysaccharide capsules. The most important cause in individuals aged 3 months to 50 years is Neisseria meningitidis (the meningococcus), a gram-negative diplococcus. Other important organisms include Streptococcus pneumoniae, Haemophilus influenzae, Listeria monocytogenes, and in neonates Streptococcus agalactiae (Group B Streptococcus).

N. meningitidis is normally a respiratory commensal in 5 to 10% of people and spreads via respiratory aerosols. Infection occurs when a non-carrier is exposed and becomes colonised. Virulence depends on pili/OPA and OPC adhesins for epithelial attachment, outer membrane porins for translocation across the blood-brain barrier, a polysaccharide capsule that inhibits phagocytosis and complement, and lipooligosaccharide (LOS) which is highly proinflammatory.

Clinical features of bacterial meningitis include fever (present in 95%), neck stiffness, headache, and altered mental state. Haemorrhagic rash (petechiae progressing to purpura and ecchymoses) is characteristic of N. meningitidis septicaemia. Diagnosis requires lumbar puncture for CSF: turbid CSF with neutrophilia, raised protein, and low glucose indicates bacterial meningitis. PCR is the most sensitive and specific test and is unaffected by prior antibiotic therapy. Treatment requires immediate empirical IV ceftriaxone before sample results. Confirmed N. meningitidis is treated with IV ceftriaxone; rifampicin is given to close contacts to eradicate carriage. Viral meningitis (aseptic meningitis) typically causes lymphocytosis in CSF and is usually self-limiting.